The present invention generally relates to optical communication systems and, more particularly, to the use of optical fibers in optical communication systems and a digitally controlled optical fiber delay line.
Optical delay lines are used ubiquitously in optical communications to arrange and control the optical data flows, synchronize optical pulses, and equalize the lengths of different channels in fiber arrays. Delay lines are employed in a time-multiplexing application, for example, in the buffers controlling the timing of signal routing, for synchronizing data bits, and for compensating dispersion and delay between different channels. In a number of cases, the application of adjustable time delays to communication channels is necessary, for example, to compensate for the influence of environmental conditions, the channel length change due to substitution of worn-out system parts, or just because of changing to an operating regime requiring different delays at different times. In addition, even a well-behaved network requires a new equalizing of the lengths of channels whenever any optical element is replaced or new splicing is produced.
The existing optical delay lines in present use can be divided into two large groups. One of the groups is based on light propagation through a free space with fixed or adjustable length. FIG. 1 illustrates an example delay line of this kind, which is disclosed in U.S. Pat. No. 5,066,088, issued Nov. 19, 1991, and incorporated by reference. Delay line 100 comprises a pair of fiber collimators 102, 104 and a reflection element 106, such as a mirror or prism, for example, placed on a motorized stage 108, which enables changing the optical path 110 from one fiber collimator, e.g. 102, to the other, e.g. 104, and so changing the delay. These devices have generally good accuracy and reliability. They typically have small dynamic range of time variation, however, which results from restricted size of the delay lines. For example, a 10 nanosecond (ns) delay corresponds to a 3 meter (m) optical path.
FIG. 2 illustrates another example delay line of this kind, which is disclosed in U.S. Pat. No. 6,147,799, issued Nov. 14, 2000, and incorporated by reference. Delay line 101 comprises a pair of fiber collimators 102, 104 and uses multiple reflections of optical path 110 from mirrors 107, 109 for dynamic range extension together with conserving the overall dimensions of the device. Nevertheless, an extension of the delay line dynamic range will lead to large overall dimensions and to an increase in the total losses. Both of these examples exhibit a low rate of switching between different delays resulting from the necessity of mechanical translation of the stage with either reflection element or collimator. To reach a higher accuracy of stage positioning, for example, a lower speed of translation is chosen, which leads to an increase in the switching time. Generally, it is not reasonable to expect a mechanical translation of such construction to give a switching frequency of more than a few Hertz (Hz). Delays lines of the kind represented by this first group are commercially available, for example, from SANTEC Corp. of Hackensack N.J., and General Photonics Corp. of Chino, Calif.
The second group of existing optical delay lines is based on light propagation through fibers. If different delays are necessary for different pulses, a multi-fiber delay line incorporating nested optical fibers is usually used. An example of such a delay line is illustrated in FIG. 3 and is disclosed in European Patent No. EP1099965, published May 16, 2001, which is incorporated by reference. Delay line 200, shown in FIG. 3, includes a plurality of optical fibers 202, each having a “unique predetermined optical length.” This kind of delay is often employed in time-multiplexing applications when the delay parameters are known and stationary. In this case, it is possible to modify the delays a little by stretching of the fibers through heating or piezoelectric expansion of the support which the optical fibers are attached to, however, it has a very restricted range of delay variation. Such a method of delay changing is disclosed in U.S. Pat. No. 6,215,941, issued Apr. 10, 2001, which is incorporated by reference. Because direct or dynamic modification of fiber length in a wide range—such as by stretching—is considered impossible, delay lines have been disclosed that use a number of fibers with slightly varying lengths for the realization of high resolution variable optical delay lines. An example of such a delay line is illustrated in FIG. 4 and is disclosed in U.S. Patent Application Publication No. US 2002/0067877, published Jun. 6, 2002, which is incorporated by reference. Delay line 201, shown in FIG. 4, includes a plurality of optical fibers designated as a fiber bundle 220. Switches between different fiber delays, i.e., between different length fibers of fiber bundle 220, are provided by using an optical micro-electromechanical mirror array 222 and a scanning mirror 224. After reflection from the end of the fiber arrays 226, the optical signal passes through the fiber delay a second time and the output 229 is separated from the input 221 with a circulator 228. This device has good response time, however, it is necessary to use a huge number of fibers to achieve delay variation over a wide range with high time resolution.
As can be seen, there is a need for an optical fiber delay line that combines a good time resolution over a wide dynamic range with the possibility of digitally controlling and fast switching the time delay. There is also a need for an optical fiber delay line that provides good time resolution over a wide dynamic range while using a minimal number and amount of optical fibers.